The present invention is in the field of galvanic coating processes, and generally relates to a galvanic bath and to a process for the generation of textured hard-chrome layers on surfaces.
It has long been current state of the art to provide objects with surface coatings by means of galvanic processes in order to impart to the objects special functional and/or decorative surface properties, such as hardness, corrosion resistance, metallic appearance, luster, or other properties. Galvanic baths that contain nickel or chromium serve in technical applications mostly for the generation of especially hard, mechanically resistant layers. In galvanic surface coating processes, the metal to be precipitated is provided in a bath in the form of a dissolved salt. The metal is deposited by means of direct current onto the object to be coated which is connected to a cathode. The object to be coated consists as a rule of a metallic material. When this is not the case, and when the object is not electrically conductive, then the surface of the object can be made conductive, for example, by a thin metallization.
In certain cases it is requisite or desired that objects that are provided with a galvanically generated hard-chrome layer present a xe2x80x9croughxe2x80x9d surface texture. In decorative coatings, for example, it may be desired to imprint a matte appearance or an agreeable, non-smooth xe2x80x9cfeel.xe2x80x9d For technical applications, rough hard-chrome layers or textured-chrome layers have certain functional properties. For example, in machine components which stand in sliding contact with one another, such as, for example, pistons, cylinders, running sleeves, axle bearings, and so forth, rough hard-chrome layers are advantageous, since the texture causes lubricant deposits, thus preventing a dry-running. In the graphics industry, for sheet-guiding drums in printing presses, for example, inking rollers and dampening rollers with a rough surface are needed. Similarly, in the shaping and bending fields, texturally chromed tools can be used in order to impart a textured surface to the work-piece to be processed.
According to conventional techniques, objects with hard-chrome coating and rough surface texture are obtained by mechanical processing, such as grinding, sandblasting, spark erosion, etc., or by chemical etching processes before, between, or after the chroming. Such processes, however, are complicated and expensive by reason of the large number of requisite different working techniques.
From DE 42 11 881 there is known a galvanic process for applying surface coatings to machine components, in which, for example, chromium is precipitated in textured form. Here by at least one initial, and at least one subsequent voltage or current impulse, as well as by a certain conducting of the voltage or current function, there is first brought about a nucleation on the surface of the machine component, and subsequently growth of the nuclei of the precipitation material is caused. The chromium is precipitated in the form of statistically uniformly distributed dendritic- or approximately hemispherical- (cap-shaped) raised parts. Another reference, EP 0 722 515, contains a further development of the process according to DE 42 11 881 in which the increase of the electrical voltage or of the current density occurs in stages.
The forgoing references disclose the use of conventional galvanic baths. In DE 34 02 554 C2 it is proposed to use a saturated sulfonic acid with at most two carbon atoms and at most six sulfonic acid groups or salts or halogen acid derivatives thereof to increase the current yield in the galvanic precipitation of hard-chrome on a work-piece of steel or aluminum alloy from an aqueous non-etching electrolyte containing chromic acid and sulfuric acid. Another document, U.S. Pat. No. 5,176,813 discloses a process for the galvanic precipitation of chromium from a galvanic bath with a lead-containing anode in the absence of monosulfonic acid, in which the galvanic bath contains chromic acid, sulfate ions and at least one halogenated alkyl polysulfonic acid or its salt with 1 to 3 carbon atoms.
Known processes in which textured chrome layers are galvanically generated suffer from disadvantages. They demand a complicated multi-layer layer-construction, in which, before the textured chrome layer proper is applied, a nickel strike-layer is first applied onto the base material of the component, then a thicker sulfate nickel layer, followed by the chrome layer, and last of all a covering with a fissure-poor hard-chrome layer. These different layers require specific, differently composed galvanic baths and different precipitation conditions, adjusted in each case to the particular layer to be deposited. The conducting of the process is therefore costly, complicated and, by reason of the necessary working steps, very cost-intensive. Further, there are obtainable via such processes only layers with roughness values Rz of up to about 10xcexc. Moreover, the uniformity of the distribution and the formation of the cap-shaped raised parts is still in need of improvement.
It is a general object of the present invention to substantially simplify the generation of textured hard-chrome layers and, in particular, to make possible textured layers with more uniform surface topography and substantially higher roughness values.
It has now been found that textured hard-chrome layers corresponding to the foregoing general object can be obtained from a galvanic bath that contains at least one chromium (VI)-ion delivery compound and is characterized in that it contains the following:
a) Chromium(VI)-ions in an amount that corresponds to 100 to 600 g/ltr of chromic acid anhydride;
b) Sulfate ions in the form of sulfuric acid and/or of a soluble salt thereof in a molar concentration ratio of chromium(VI)-ions to sulfate ions (SO4xe2x88x922) of 90:1 to 120:1, and
c) 2-hydroxyethane sulfonate ions in an amount to corresponds to 0.01 to 3.0 g/ltr of the sodium salt.
It has been found, surprisingly, that the inventive combination of the components sulfate and 2-hydroxyethane sulfonate results in especially advantageous properties of the chrome bath.